An anemometer is a device which measures the velocity and direction of gas flow. A Wheatstone bridge can be used as an anemometer. A Wheatstone bridge comprises four resistances connected together in a square configuration, with two pairs of parallel connecting legs forming the sides of the square, and four electrically conductive contacts located at the corners. Application of a known voltage between two diagonally opposed corner contacts results in a voltage reading on a meter connected across the other diagonally opposed corner contacts.
A Wheatstone bridge with four resistances of known value can be used as a sensor to measure parameters such as pressure, force, flow rate and direction. Such a Wheatstone bridge is symmetrical and, in principal, remains in balance for any ambient temperature. However, gas or other mass flow across the bridge cools the legs that are perpendicular to the flow. Because resistivity of most materials is temperature dependent, the flow affects the resistance of these legs, sets the bridge into imbalance and results in an voltage change corresponding to the velocity of the flow. Generally, the resistors most affected by the air flow will be the resistors that are oriented transverse to the direction of the air flow, the resistors whose entire length is exposed to the flow. However, the resistors oriented parallel to the flow will also be somewhat affected, depending upon the aspect ratio of the resistor legs. The aspect ratio is the ratio of the length to the width of each resistor leg. The sensitivity of such a device increases as the aspect ratio increases. Thus, for a Wheatstone bridge with legs of a predetermined length, sensitivity can be increased by decreasing the width of the legs.
An article entitled "Integrated Silicon Anemometer" authored by A. F. P. Van Putten and published in Vol. 10, No. 21 of Electronics Letters on Oct. 7, 1974, discloses a solid state silicon anemometer. This anemometer provides faster response time than prior "hot wire" anemometers, due to its smaller size. Briefly, a solid state silicon anemometer of this type consists of a planar 1.5.times.1.5 millimeter chip of n-type silicon with 4 p-type diffused regions forming the resistor legs of the Wheatstone bridge. Shallow impurities such as boron and phosphorus were used to achieve the n- and p-type doping, respectively. The resistors were connected at the corners by evaporated aluminum contacts.
There are several inherent problems with a solid state anemometer of this type. Because the voltage drops along the length of each resistor leg, the voltage differential at the isolation depletion region between the p-type material and the n-type material along the length also varies correspondingly. This causes the cross-sectional dimension of each resistor leg to be non-uniform along the length, resulting in non-uniform heating of the leg. This problem was somewhat overcome by extending the contacts in parallel conductor pairs along each of the legs, with each conductor connected at one of its ends to one of the corner contacts and a resistor leg residing between each conductor pair. This configuration produces voltage drop in a direction transverse to the orientation of the resistor leg, and also results in much lower bridge impedance, which may or may not be desirable.
Unfortunately, other more serious problems associated with devices of this type are not solved as easily. First, the high heat transfer rate of silicon (higher than that of most steels), tends to thermally short out the temperature differential between the parallel and the perpendicular bridge legs. With no measurable temperature differential, there is no differential change in resistance and no signal output to indicate flow rate. Thus, once heated, this solid state anemometer loses sensitivity to heat flow.
Secondly, because of the relatively large thermal mass of the silicon chip, a slight change in heat due to air flow must last long enough to overcome or impact upon the resistivity of the resistor leg, or it will go undetected altogether. Consequently, the time response for this circuit is quite long, typically in the range of a minute to several minutes, and flow variations of short duration typically go undetected. The response time for other microelectronic circuits is generally much faster, and thus this relatively slow response time hinders circuit compatibility with these devices. While further miniaturization of the device would speed up response time, further miniaturization would also compound the problem of thermal shorting, and thus miniaturization results in a further decrease in flow sensitivity. Further miniaturization also compounds problems associated with electrical shorting. Under the applied voltage, some current will leak between adjacent contacts via a route other than through the resistor legs, thereby affecting the relationship between the applied voltage and the measured voltage.
Finally, within the normal temperature range of most operative microelectronic circuits, the free carrier concentration of semiconductor devices doped with a shallow level impurity is substantially fixed, and does not change with temperature variation. Thus, the only factor influencing the temperature coefficient of resistance is the carrier mobility, which classically varies approximately as the .sup.- (3/2) power of temperature. Therefore, the bridge leg resistance R may be expressed as: EQU R=CT.sup.3/2 Eq. (1)
where C is the constant of proportionality. Because the temperature coefficient of resistance is small, the flow sensitivity of each of these resistor legs is small. In other words, due to thermal shorting, the relatively large thermal mass of typical semiconductor crystals, fixed free carrier concentrations and to some extent, electrical shorting, solid state anemometers have achieved only a limited degree of success.